Pregnant women should take prescription flu medicines if they are diagnosed with the new swine flu, health officials said Tuesday.

So far, the swine flu has not proven to be much more dangerous than seasonal influenza, and it’s not clear whether or not pregnant women catch swine flu more often than other people. But in general, flu poses added risks for pregnant women, said Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention.

Pregnancy weakens a woman’s immune system, so that she’s more likely to suffer pneumonia when she catches the flu. In earlier flu pandemics, infection also raised the risk of a premature birth, said Schuchat.

So far, some 20 pregnant women have contracted the novel H1N1 virus now confirmed in more than 6,000 around the world. Three deaths have been confirmed in the United States. One of the three was a Texas woman who was pregnant. Her baby was delivered via Caesarean section while she was hospitalized.

Pregnant women with asthma and some other health conditions are particularly at risk for complications.

Risks from the virus are greater than the unknown risks to the fetus from the drugs Tamiflu and Relenza, Schuchat said at a press conference Tuesday.

“We really want to get the word out about the likely benefits of prompt antiviral treatment” for pregnant women, she said. CDC officials recommend Tamiflu for pregnant women.

Antiviral medications such as Tamiflu and Relenza can help lessen or avert complications, she said.

Most H1N1 flu patients do not require antiviral therapy to recover, a World Health Organization expert said on Tuesday, so saving such stockpiles for pregnant women and patients with underlying health conditions such as cardiovascular disease or diabetes may prove most prudent.

"We will recommend to consider the use of antivirals for high risk groups," said Nikki Shindo, a medical officer with the WHO's global flu program.

Still, the flu medicines’ effectiveness is somewhat limited, studies have shown. They can relieve symptoms and shorten the disease by about a day. They only work if started within 48 hours of first symptoms, and little is known about whether they cut the chances of serious flu complications. Most people recover from the flu with no medical treatment.

But a 33-year-old pregnant Texas woman who had swine flu died last week, after giving birth through an emergency cesarean section. At least 20 other pregnant women have swine flu, including three who were hospitalized.

Pregnant women with confirmed or suspected cases of swine flu should take the antiviral medicines for five days, the CDC recommends.

In total, over 4,000 U.S. cases of swine flu have been confirmed through lab testing so far, most of them ages 18 and under. Officials think the actual number of infections is much higher, and that infections are still occurring.

CDC officials said the swine flu may seem to be mild now, but they worry the virus will mutate into something more dangerous. One concern is that it will combine with the more deadly but less easily spread bird flu virus that has been circulating in Asia and other parts of the world.

Another concern is that it will combine with the seasonal H1N1 virus that went around over the winter. That virus was not unusually virulent, but it was resistant to Tamiflu — the current first-line defense against the new swine flu. If the two virus strains combine, it’s possible the swine flu will become resistant to Tamiflu as well, health officials worry.

Although it remains relatively mild in the U.S., the virus is spreading so rapidly that state health officials may soon stop counting individual cases. The H1N1 virus accounted for 40 percent of flu viruses logged in the U.S. in the past week and helped propel an uptick in overall flu-like illnesses, Schuchat said in a briefing Monday.

“I think the cases we’re confirming are the tip of the iceberg here,” she said.

“They tell us for sure this virus is circulating throughout the United States and it’s likely to be in every state,” Schuchat said, adding, “It’s a time when we really need to guard against complacency as we move to a new normal.”

Symptoms similar to seasonal fluThe CDC has started tracking the novel virus using the surveillance system used for seasonal influenza, called FluView.

Of the three confirmed deaths from complications of swine flu in the U.S. — a man in his 30s, a toddler and a pregnant woman — each suffered from several other illnesses when they were infected with the virus.

Last week, the CDC also described the symptoms experienced by Americans with swine flu. About 90 percent reported fever, 84 percent reported cough and 61 percent reported a sore throat — all similar to what’s seen with seasonal flu. But about one in four cases have also involved either vomiting or diarrhea, which is not typical for the normal flu bug.